1. Field of the Invention
The present invention relates to the production of pumpable coal-water suspensions in one process stage.
2. Discussion of Prior Art
Coal-water suspensions serve not only to transport pulverulent coal, but can also be used as a fuel for direct combustion in power-stations. Compared with coal-dust they have the advantage that the individual burners can be uniformly charged and the fuel line can be metered and monitored.
A further area of use for coal-water suspensions which is becoming of increasing importance is the gasification of hard coal or brown coal (lignite), i.e., its partial combustion with oxygen or oxygen-containing gases in the presence of water to form mixtures of carbon monoxide and hydrogen.
In this, fine-grained or pulverulent coal is converted at temperatures of about 900.degree. to about 2000.degree. C., preferably 1100.degree. to 1600.degree. C. and under elevated pressures of up to 200 bars, preferably 5 to 100 bars. Operating with fine-grained coal is particularly advantageous because modern mechanized coal mining methods have given rise to an increasing proportion of pulverulent coal. A further advantage is that pulverulent coal of practically any quality can be converted into synthesis gas irrespective of its tendency to cake and its ash content.
A typical example of a coal gasification process using fine-grained coal suspended in water is described in German Pat. No. 20 44 310. In this process coal is pre-ground dry in a mill and led to a suspension vessel. A stable, pumpable suspension is produced by adding fresh water and circulation water. This suspension is continuously pumped under the pressure of the gasification process to the burner and converted into carbon monoxide and hydrogen. Slag is formed as a by-product.
In the interests of higher energy yield in the form of carbon monoxide and hydrogen as well as process steam and a low oxygen consumption, special requirements are placed on the properties of suspensions intended to be used in coal gasification plants.
It is particularly important for the solids level of the suspension to be as high as possible, since this insures that the amount of energy to be supplied autothermally for heating that proportion of the water not involved in the reaction but only serving to transport the solids particles remains small. Furthermore, it is essential that the particle size of the solid material is sufficiently fine in order to insure its rapid conversion. This latter requirement conflicts with the fact that the viscosity of the suspension rises with decreasing particle size but must not exceed certain limiting values if the suspension is to be satisfactorily conveyed.
Coal-water suspensions which can be used in coal gasification processes are already known. Thus, suspensions are described in German Offenlegungschrift 28 36 440 which contain up to 75% by weight of solids and consist of solids particles the major proportion of which has a size of between 50 500 .mu.m. Such suspensions have indeed been successfully employed in coal gasification processes, but do not satisfy all the above-mentioned requirements. In particular, they do not enable an almost complete conversion of all the coal contained in the suspension to be achieved.
It is also known to grind particulate coal in the presence of the required amount of water in a single pass in order to produce coal-water suspensions. Suitable grinding equipment that can be used for this purpose includes various packed mills, such as tube mills or ball mills. According to a process described in German Pat. No. 15 26 174, in order to increase the grinding output of the mills the grinding is carried out in the presence of 50 to 65% by weight of water and the coal is ground to a fineness of 100% below approximately 1.5 mm. The suspension is then partially dehydrated so that the water content is 35 to 45% by weight.
The dehydration of the ground product following the grinding process, until the desired solids concentration is reached, requires additional process stages. It is thus very costly not only as regards the necessary apparatus but also as regards the necessary manpower requirements.